Constant force centralizer

ABSTRACT

A constant force device has at least a first non-constant axial force driving the first set of arms and a second non-constant axial force driving the second set of arms, where the two sets of arms are offset from one another by 90°. Each of the non-constant axial forces is converted to a radially extending force by the interaction of a force guide and actuator. The force guide is attached to the inner mandrel of the constant force device and is shaped to produce an essentially constant radially extending force through the entire range of motion of the arms. Typically each arm of the pair of arms has a pivoting arm and a telescoping arm where the joint between the pivoting arm and telescoping arm has one or more wheels to reduce friction as the constant force device moves through the tubular. Generally the first pair of arms is opposed to and overlaps by some distance the second pair of arms where the second pair of arms is 90° offset from the first pair of arms. Additional features may include friction reducing members at the joint between the telescoping arm and the pivoting arm, an extension lock, extension limiters, and rotating force guides.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 62/144,657 that was filed on Apr. 8, 2015.

FIELD OF INVENTION

The present invention relates to a device that may apply a vectored force radially outward from a central axis. Such vectored force may be applied in multiple directions at once where the application of the vectored force is to maintain the central axis of the device relatively aligned with the central axis of the tubular through which the device passes.

BACKGROUND

Once a hydrocarbon bearing well has been drilled it is usually necessary to perform several tests upon the well, for instance to determine the integrity of the casing after it has been installed, to determine for instance the quality of the cementing job, or to determine the presence and locations of any hydrocarbons adjacent to the well. Such testing is usually done with a set of instruments referred to as a logging tool. In most instances the logging tool is lowered into the well on a cable, where the cable may include a power and/or data line. Logging tools may be transported through any tubular structure including pipelines and refineries.

Certain types of logging tools work best when they are centrally positioned within the tubular structure being tested. In order to centrally position the logging tool within the tubular, a centralizer may be used. Centralizers typically use a set of springs, such as bow springs, to apply force radially outward from a central axis. Provided that the force is applied equally in all directions and that there is sufficient force to overcome any bias due to the weight of the logging tool, the logging tool will remain more or less centralized within the wellbore, whether open hole or cased hole. Unfortunately the diameter of the wellbore varies as the tool progresses through the wellbore. Variations in diameter may be due to other tools or equipment located in the wellbore or to different sizes of casing installed as the well progresses from the surface to the well's final depth. Other variations in the well diameter may be due to changes in the well's direction causing the casing to become ovalized as the tubular bends through turns. Unfortunately the force applied to different sizes of tubulars by a standard centralizer varies such that a centralizer may have sufficient force to keep a logging tool centralized in one size of tubular, but when the logging tool is in a smaller diameter tubular such force is excessive, causing damage to the centralizer or even preventing the centralizer from progressing through the well. On the other hand while the force applied may be sufficient to keep a logging tool centralized in one size of tubular, in a larger diameter tubular such force is inadequate allowing the logging tool to substantially deviate from the center of the tubular.

In order to address such concerns many variations of constant force centralizers have been developed. There are several constant force centralizers available in the market, but there is very little information showing quantitative force values vs. casing size. Ideally, each constant force centralizer would have a force chart similar to the force chart shown in FIG. 1.

Though customers seem to have a clear need for a constant force centralizer, such requests do not appear to include a definition of “constant.” The understanding is that clients just need a device that keeps their tools centralized in a wide range of environments.

SUMMARY

A constant force centralizer is envisioned where a first non-constant axial force drives the first set of arm assemblies and at least a second non-constant axial force drives the second set arm assemblies where the two sets of arm assemblies are offset from one another by 90°. Typically the non-constant axial forces are provided by some type of biasing device usually a spring or compressed gas but other types of biasing devices may be used. A force guide may be permanently affixed, rotatably attached, or otherwise mounted on the central mandrel of the constant force centralizer. Each of the non-constant axial forces is converted to a radially extending force by an interaction of a three guide and actuator. The force guide is shaped to produce an essentially constant radially extending force through the entire range of motion of the arm assemblies. Preferably the radially extending force is maintained throughout each arm assembly's travel within about ten percent of the maximum radially extending force. Typically, each arm assembly is comprised of a pivoting arm and telescopic section. Typically a wheel is positioned at the joint of the pivoting arm and the telescopic section to reduce friction as the constant force centralizer moves through the tubular. In the collapsed condition where the pivoting arm and wheel are relatively close to the mandrel of the constant force centralizer the telescoping arm is in its substantially shortest state whereas in the extended condition where the pivoting arm and wheel are at their maximum distance from the mandrel the telescoping arm is in its longest state. The telescoping arm is generally necessary in order to allow the constant force centralizer to reverse direction when moving from a larger diameter tubular to a smaller diameter tubular. A portion of the telescoping arm will interact with the tubular to force the pivoting arm and wheel to retract to at least a semi-collapsed condition. By utilizing a telescoping arm in place of a solid arm, the overall length of the constant force centralizer is shorter than would otherwise be possible, and this is considered beneficial for many logging tool embodiments.

It is envisioned that two pairs of arm assemblies will usually be used in a constant force centralizer. The pairs of arm assemblies are typically arranged such that a first end of the first pair and a first end of the second pair of arm assemblies extend toward each other from a first end of a mandrel and from an opposing second end of the mandrel. Generally the first pair of arm assemblies is allowed to collapse into a nested position with the second pair of arm assemblies. When fully collapsed, opposing pairs of pivoting arms overlap by some distance. The overlap and telescoping arms generally allows the tool to be shorter than a standard tool not having overlapping arms.

Ovalized casing is encountered occasionally, and centralization in such conditions can be difficult. Constant force centralizers offered to date have linked arms providing lateral arm movement that is symmetric in all directions. In round casing and in vertical wells, this arrangement is adequate. However, in deviated wells where the casing is ovalized, these centralizers may not perform well. In a current embodiment typically the arms that are offset from one another at some angle, typically 90°, allow for the offset arms to provide non-symmetric arm movement in at least two directions providing centralization even in non-symmetric or ovalized wellbores or tubulars. In certain situations it has been found that non-symmetric constant force is necessary such that the tool is held in an eccentric condition within the well.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 depicts a calculated and measured force curve of an embodiment of the invention.

FIG. 2 depicts a calculated force curve of an alternate embodiment of the invention.

FIG. 3 depicts a side view of an embodiment of the invention in its extended condition.

FIG. 4 depicts an end view of an embodiment of the invention.

FIG. 5 depicts a side view of an embodiment of the invention in its retracted condition.

FIG. 6 depicts an end view of an embodiment of the invention in a partially extended condition in an oval tubular.

FIG. 7 depicts an extended joint of an embodiment of the invention.

FIG. 8 depicts a side view of an alternate embodiment of the invention in its extended condition.

FIG. 9 depicts a side view of an alternate embodiment of the invention having a rotatable force guide in an extended condition of a constant force centralizer.

FIG. 10 depicts an orthogonal view of a rotatable force guide.

FIG. 11 depicts a side view of an alternate embodiment of the invention having an extension limiter in a limited extension condition.

FIG. 12 depicts a side view of an alternate embodiment of the invention having an extension lock in a retracted and locked condition.

FIG. 13 depicts a close-up of the area A from FIG. 12.

DESCRIPTION

FIG. 1 depicts a graph of the measured force curve 10 versus the predicted force curve 14 of an embodiment of the present invention. The measured force curve 10 is a poly fit of the measured points 12 while the predicted force curve is a based upon a computer simulation. A perfectly flat, linear response was the original design goal, but in order to keep the mechanisms relatively simple, a slight “curve”, as depicted by the predicted force curve 14 and the measured force curve 10 was thought to be acceptable.

FIG. 2 depicts a graph of the predicted force curve 20 of an alternate embodiment of the present invention. While other force ranges may be used, the predicted force curve 20 utilizes a force range of from about 40 pounds of force at the minimum diameter of just over three inches increasing to about 43 pounds of force at the mid-range diameter of 8 inches then decreasing again to about 40 pounds of force at the maximum diameter of about thirteen inches. Such a force range has less than a 10% variation across the range of applied force from the minimum diameter to the maximum diameter.

FIG. 3 is a side depiction of an embodiment of a constant force centralizer 50 providing a substantially constant radially outward force throughout a predetermined range of tubular diameters. The constant force centralizer 50 has an inner mandrel 52, beginning with the right side of the constant force centralizer 50, and at least one axial biasing device such as axial biasing device 54. A collar 58 is fitted to the mandrel 52 in such a manner that its position is fixed relative to the mandrel 52. The collar 58 may be threaded, pinned, welded, or formed as an integral part of inner mandrel 52, or connected by any other means known to the inner mandrel 52. The axial biasing device 54 typically surrounds inner mandrel 52 and abuts collar 58. The axial biasing device 54 also abuts a movable sleeve 62. Typically the movable sleeve 62 is circumferential about an exterior surface of inner mandrel 52. The movable sleeve 62 is generally only axially movable. A first end 70 and 72 of pivotal force arms 64 and 68 is attached to movable sleeve 62. Each pivotal force arm 64 and 68 has a recess 74 and 76. Within each recess 74 and 76 is an actuator 80 and 82, such as a roller. Each recess 74 and 76 is sized such that when pivotal force arms 64 and 68 are in the retracted position, lying flat against inner mandrel 52, most of the force guides 84 and 88 that extend beyond the exterior surface of inner mandrel 52 are contained within each recess 74 and 76. The force guides 84 and 88 are fixed to the inner mandrel 52 and maybe threaded on, pinned on, or formed as an integral part of the inner mandrel 52. It is generally the interaction between the force guides 88 and 84 with the corresponding actuators 80 and 82 that describes the constancy of the force curves such as the curves in FIGS. 1 and 2. Each force guide 88 and 84 will have a surface such as surfaces 90 and 92. Generally the surfaces 90 and 92 are linear surfaces at some angle α relative to the axis of mandrel 52 where the angle α provides a reasonably flat force curve. The angle α in FIG. 3 is 47°.

Continuing with the left side of the constant force centralizer 50, the constant force centralizer 50 has at least one axial biasing device such as axial biasing device 56. A collar 100 is fixed onto a second end 102 of inner mandrel 52. The collar 100 is typically threaded onto inner mandrel 52 but may be pinned, welded, or formed as an integral part of inner mandrel 52. The axial biasing device 56 typically surrounds inner mandrel 52 and abuts collar 100. The axial biasing device 56 also abuts a movable sleeve 104. Typically the movable sleeve 104 is circumferential about an exterior surface of inner mandrel 52. The movable sleeve 104 is generally only axially movable. A second end 106 and 108 of telescopic arms 110 and 112 is attached to movable sleeve 104.

A first end 114 and 116 of telescopic arms 110 and 112 is pivotally connected to a second end 120 and 122 of pivotal force arms 64 and 68. Generally, at the pivotal connection where first end 114 and second end 122 as well as first end 116 and second end 120 are connected, a wheel, such as wheel 124 and 126, a roller, a skid, or other friction reducer is attached. Generally it is at wheels 124 and 126 that the constant force is applied to the casing or other tubular in a direction perpendicular to the long axis of the constant force centralizer 50.

When the constant force centralizer is in a tubular with sufficiently small diameter, each of the pivotal force arms 64, 68, and telescopic arms 110, and 112 will be in a collapsed position such that wheels 124 and 126 are at a minimal radial distance from inner mandrel 52. With wheels 124 and 126 at their minimal radial distance from inner mandrel 52, axial biasing device 54 is at maximum compression thereby applying the maximum normal force against movable sleeve 62. The force applied by axial biasing device 54 is transferred to the movable sleeve 62. The force applied by axial biasing device 54 is not necessarily constant. Movable sleeve 62 in turn transfers the force to pivotal force arms 64 and 68. Subsequent movement of pivotal force arm 64 is guided by actuator 80 acting on surface 90 causing end 122 to move in a direction substantially perpendicular to the axis of the mandrel 52. The dimensions of pivotal force arm 64, actuator 80, force guide 88, movable sleeve 62, collar 58 and biasing device 54 are chosen so that the force from the biasing device 54 is transferred to the wheel 124 in such manner that that force of wheel 124 against the tubular remains reasonably constant as the diameter of the tubular changes. Movement of pivotal force arm 68 is guided by actuator 82 acting on surface 92 causing end 120 to move in a direction substantially perpendicular to the axis of the mandrel 52. The dimensions of pivotal force arm 68, actuator 82, force guide 84, movable sleeve 62, collar 58 and biasing device 54 are chosen so that the force from the biasing device 54 is transferred to the wheel 126 in such manner that that force of wheel 126 against the tubular remains reasonably constant as the diameter of the tubular changes.

When the constant force centralizer 50 moves from a large diameter tubular to a smaller diameter tubular the force vectors are reversed such that the wheels 124 and 126 are forced inward exerting force through the pivotal arm 64 and 68 to the actuators 80 and 82 were the forces are redirected by the interaction of the actuators 80 and 82 with force guides 84 and 88 into movable sleeve 62 and ultimately into axial biasing device 54.

As indicated in FIG. 2, a current embodiment of the tool produces 40 lbs of radial force at the wheels 124 and 126 at the joint between pivoting force arms 64 and 68 and telescoping arms 110 and 112. The force response is adjustable with the nominal radial force either increased or decreased. Such increases or decreases may be adjusted where biasing devices 54 and 56 may be replaced with springs, gas chambers, etc. having proportionally higher or lower force rates. If needed, shims can add compression to the biasing devices 54 and 56 and thereby increase the axial force. A flatter force response curve, see FIGS. 1 and 2, is achievable if a more complex shape is machined into the force guides 84 and 88.

As further indicated in FIG. 3, the force guides 84 and 88 are generally fixed to the inner mandrel 52 of the constant force centralizer 50. In a current embodiment. the force guides 84 and 88 have an angle α where the angle α is about 47° relative to the axis of the inner mandrel 52 achieving a substantially constant force response as indicated in FIG. 2. The force guide angle and/or shape controls the shape of the force response curves such as the force response curves in FIGS. 1 and 2.

FIG. 4 is an end view of the fully collapsed constant force centralizer 50. In the embodiment of the constant force centralizer depicted an outside diameter of 3.5 inches was chosen as the nominal diameter of the centralizer. A smaller outer diameter can be achieved in the centralizer design by scaling down the size of the components.

In general, for wireline tools, shorter tools are preferred. As tools become shorter, their overall weight is reduced. In one embodiment of the 3.5 inch diameter constant force centralizer 100, the total tool weight is less than 40 pounds. With this in mind, as indicated below, several unique design features were employed to minimize the tool length. As shown in FIG. 5, one embodiment of the constant force centralizer 100 has a length of 26.8 inches. The relatively short tool length of the design is a result of offsetting the pivoting force arms 102, 104, and 106. The pivoting force arm 102 is attached to movable sleeve 108 while pivoting force arm 104 is attached to movable sleeve 110 by pin 112 and pivoting force arm 106 and is attached to movable sleeve 110 by pin 114. The pivoting force arm 102 is connected to telescoping arm 122 at the joint 132. Also at joint 132 are wheels 116 and 117. Telescoping arm 122 has a first portion 124, connected to pivoting force arm 102 at joint 132, and a second portion 126. Second portion 126 is attached to movable sleeve 110 via pin 134. In this embodiment the second portion 126 slides within the first portion 124. The other two pivoting force arms 104 and 106 seen in FIG. 5 are each rotated 90° around the central axis of the constant force centralizer 100. For ease of reference only pivoting force arm 104 will be further described. As described previously pivoting force arm 104 is attached to movable sleeve 110 by pin 112. The pivoting force arm 104 is connected to telescoping arm 128 at the joint (not shown) where wheel 118 is attached to the constant force centralizer 100.

As can be seen in FIG. 5 the movement of the arms occurs in two planes (not shown). The two planes are perpendicular to each other and both planes contain the axis of the constant force centralizer 100. Additionally each of the wheels 116 and 118 are offset by some axial distance D. The distance D may vary depending upon whether the arms are fully extended or fully collapsed or at some point in between.

When the arms are fully open the wheels 116 and 118 are axially offset by 1.35 inches. When the arms are fully closed the wheels 116 and 118 are axially offset by 2.1 inches. With offset wheels, the centralizer 100 can traverse radial upsets in the tubular more easily, and erratic tool movement is minimized.

Generally by having the telescoping arms 122 and 128 attached to their respective pivoting force arms 102 and 104 the respective axial biasing devices 140 and 142 operate to apply force to their associated wheels 116 and 118 independently.

The wheels 116, 118, 117 and 120, at each of the joints between the pivoting force arms 102, 104, and 106 and the telescoping arms 122, 128, and 130 are free to rotate even when the tool is completely closed to its minimum outside diameter. In the embodiment of the constant force referred to in FIG. 2 the constant force centralizer is designed to open from about 3.5 inches to about 12.7″ which is the inner diameter of typical casing that has an outer diameter of 13⅜ inches. As shown in FIG. 2, about 40 pounds of centralizing force is active across that entire range.

In an embodiment of the current invention of the constant force centralizer 100 from FIG. 5 as further depicted in FIG. 6 the pivoting force arm 102 is paired with the pivoting force arm 107 on the opposite side of the constant force centralizer 100. The opposing pivoting force arms 102 and 107 move symmetrically with one another. The telescoping arm 128 is paired with the telescoping arm 130 on the opposite side of the constant force centralizer 100. The opposing telescoping arms 128 and 130 move symmetrically with one another.

The telescoping arms 128 and 130 arms are generally orthogonal to the pivoting force arms 102 and 107. The pivoting force arms 102 and 107 are typically coupled to each other such that the axial biasing device 140 drives both of the pivoting force arms 102 and 107. While the telescoping force arms 128 and 130 may be linked to the same movable sleeve 108 as the pivoting force arms 102 and 107 the telescoping mechanism does not allow force to be applied by movable sleeve 108 to the telescoping force arms 128 and 130. In oval holes, conventional wisdom suggests that one pair of arms, either the pivoting force arms 102 and 107 or the telescoping arms 128 and 130, will naturally align with the “long axis” of the hole. In FIG. 6 the long axis of the tubular 136 is depicted as being 12.4 inches as shown by reference numeral 133 while the short axis of the tubular 136 is depicted as being 11.3 inches as shown by reference numeral 135. The position of the constant force centralizer as depicted in FIG. 6 is preferable and is likely to maintain good tool centralization in oval holes.

Several features of the constant force centralizer are intended to minimize rolling friction. The wheels 150 and 152, as depicted in FIG. 7, in an embodiment of the constant force centralizer are preferably as large in diameter as possible, here 1.3 inches in diameter as shown by reference numeral 154, without exceeding the desired 3.5 inch constant force centralizer outside diameter. Maximizing the wheel diameter allows each wheel 150 and 152 to last longer and roll more smoothly across irregularities in the tubular. Typically, each joint between the telescoping arm 158 and the pivoting arm 156 arm has two wheels 150 and 152 at the joint. Each wheel rolls independently on ball bearings 160.

As shown in FIG. 8, when an embodiment of the constant force centralizer 200 is fully open, the pivoting force arms 202 and 204 are at an angle β relative to the axis of the constant force centralizer 200. In this instance angle β is 30°. The telescoping arms 206 and 208 are at an angle Ω relative to the axis of the constant force centralizer 200. In this instance angle Ω is 35°. Generally, it is desired to have the angles β and Ω as shallow as possible in order to help the constant force centralizer 200 slide through any restrictions that may exist within the tubular.

FIG. 9 is a depiction an alternative embodiment of the constant force centralizer 300. In some instances it has been found desirable to allow the inner mandrel 302 to remain fixed to the wireline or other transporting device while allowing the components of the centralizer assembly including the rotatable force guide 304, pivoting force arms 306, telescoping arms 308, first axial biasing device 312, first movable sleeve 316, second axial biasing device 314, second movable sleeve 318, and other associated portions of the centralizer assembly to rotate around the inner mandrel 302. By allowing the centralizer assembly to rotate around the inner mandrel 302 the wireline (not shown) avoids becoming twisted thereby avoiding any torque build up on account of constant force centralizer 300.

FIG. 10 is a depiction of the rotatable force guide 304 from FIG. 9. The rotatable force guide 304 typically consists of a first-half 356 and a second half 358. Each half 356 and 358 has a semicircular section such as 366 and semicircular section 364 each half 356 and 358 also has at least a portion of the force guide 304 attached to the semicircular sections 366 and 364. The upper force guide includes a relatively linear surface 370 set at an angle α to the central axis of the inner mandrel 302 of the constant force centralizer 300. The upper force guide also includes a means to pivotally attach a limiting arm (not shown) such as providing a slot 372 for a wrist pin (not shown). The lower force guide includes a relatively linear surface 368 set at an angle α to the central axis of the inner mandrel 302 of the constant force centralizer 300. The lower force guide also includes a means to pivotally attach a limiting arm (not shown) such as providing a slot 374 for a wrist pin (not shown).

In the embodiment shown the rotatable force guide 304 is applied to the inner mandrel 302 by placing each half 356 and 358 such that the semicircular portions 364 and 366 surround the inner mandrel 302. Then using bolts such as bolts 362 and 360 to fix each half 356 and 358 in place around inner mandrel 302. It is envisioned that any known means of manufacturing a rotatable force guide could be used for instance in some instances the force guide 304 could be machined out of a solid piece of material and then slid onto the mandrel 302 from one end.

FIG. 11 is a depiction an alternative embodiment of the constant force centralizer 400. In some instances it has been found desirable to limit the outward translation of the pivoting force arm 402 in turn limiting the outward translation of the telescoping force arm 404 and wheel 406. Such a limitation may be useful in, for instance, circumstances where the constant force centralizer 400 may pass through very large openings such as when it passes through a blowout preventer which might cause damage to the constant force centralizer 400.

One such extension limiter may use a link 410 attached to the inner mandrel 412 or as is shown in FIG. 11 a first end of link 410 is attached to the rotatable force guide 414 by wrist pin 416 within slot 472. A second end of link 410 is attached to pivoting force arm 402 by wrist pin 418 within slot 420. Wrist pin 418 is configured such that it may slide within slot 420 depending upon the extension position of wheel 406 as wheel 406 moves towards inner mandrel 412 wrist pin 418 will move towards wheel 406 within slot 420. However as wheel 406 moves away from inner mandrel 412 wrist pin 418 moves within slot 420 towards movable sleeve 422. Eventually wrist pin 418 reaches the end of slot 420 closest to movable sleeve 422 whereupon wheel 406 is prevented from moving any further radially outward from inner mandrel 412.

FIG. 12 is a depiction of an alternative embodiment of a portion of a constant force centralizer 500. In most instances it has been found to be preferable to restrict any expansion of the force pivoting arms 502, 504, and 506 as well as the associated telescoping arms 508, 510, and 512 until at least the constant force centralizer 500 has been deployed into the tubular or wellbore. Preferably a lock will maintain the pivoting force arms and telescoping arms in the retracted position until some predetermined parameter is reached. For instance a pressure actuated retaining pin 520 may be used where the pressure actuated retaining pin 520 is designed to protrude from the force guide 522 when the constant force centralizer 500 is below some preset pressures such as atmospheric pressure. The portion of the pressure actuated retaining pin 520 that protrudes from the force guide 522 engages pivoting force arm 502 and prevents it from opening. When the constant force centralizer 500 enters the tubular the external pressure may be increased such that at some predictable point the pressure will be sufficient to force the pressure actuated retaining pin 520 inward into its recess within the force guide 522. With the pressure actuated retaining pin 520 moved inward the pivoting force arm 502 is released so that the wheel 524 may move radially outward to engage the tubular at the predetermined force level.

FIG. 13 is section A from FIG. 12. FIG. 13 depicts force guide 522 having the pressure actuated retaining pin 520 within recess 524. In the embodiment shown in FIG. 13 a pressure actuated retaining pin is utilized. In other instances the retaining pin could be actuated by temperature, elapsed time, a sacrificial wear pin, by a chemical reaction, or by an electrical signal. A portion 526 of the pressure actuated retaining pin 520 extends from force guide 522 into a port 528 within pivoting force arm 502. The pressure actuated retaining pin 520 and recess 524 form a chamber 530 sufficient to allow the pressure actuated retaining pin 522 to move into the recess 524 within force guide 522 upon the application of sufficient force to port 528 and acting upon the portion of the pressure actuated retaining pin 522 that extends into port 528. The pressure actuated retaining pin 522 may be held outwardly extended by the force exerted upon the pressure actuated retaining pin 522 by the pivoting force arm 502 and/or may have any other means known in the industry for securing the pressure actuated retaining pin 522.

While the embodiments are described with reference to various implementations and exploitations, it will be understood that these embodiments are illustrative and that the scope of the inventive subject matter is not limited to them. Many variations, modifications, additions and improvements are possible. Variations are likely to be beneficial when employed in tools such as calipers, anchoring devices, eccentering devices, and downhole tractors.

While the embodiments shown are described with the intention of maintaining a substantially constant radial force across the full operating range of the device, it is understood that, if desired, the mechanism can be modified to achieve different radial forces within different size tubulars.

Plural instances may be provided for components, operations or structures described herein as a single instance. In general, structures and functionality presented as separate components in the exemplary configurations may be implemented as a combined structure or component. Similarly, structures and functionality presented as a single component may be implemented as separate components. These and other variations, modifications, additions, and improvements may fall within the scope of the inventive subject matter. 

What is claimed is:
 1. A constant force tool comprising, a pivoting force arm, an axial biasing device, wherein the axial biasing device exerts an axially directed force upon the pivoting force arm, a force guide mounted on a mandrel, wherein the force guide interacts with the pivoting force arm to convert the axially directed force to a radially directed force.
 2. The constant force tool of claim 1 further comprising a telescoping arm wherein a first end of the telescoping arm is pivotally attached to the pivoting force arm.
 3. The constant force tool of claim 2 further comprising at least one wheel mounted at the pivotal attachment between the pivoting force arm and the telescoping arm.
 4. The constant force tool of claim 1 wherein, the axial biasing device is a spring.
 5. The constant force tool of claim 1 wherein, the radially directed force is substantially constant.
 6. The constant force tool of claim 1 wherein, the radially directed force is constant within about 10% of the maximum radially directed force.
 7. A constant force tool comprising, a pivoting force arm, an axial biasing device, wherein the axial biasing device exerts an axially directed force upon the pivoting force arm, a force guide rotatably mounted on a mandrel, wherein the force guide interacts with the pivoting force arm to convert the axially directed force to radially a directed force.
 8. The constant force tool of claim 7 further comprising a telescoping arm wherein a first end of the telescoping arm is pivotally attached to the pivoting force arm.
 9. The constant force tool of claim 8 further comprising a wheel mounted at the pivotal attachment between the pivoting force arm and the telescoping arm.
 10. The constant force tool of claim 7 wherein, the axial biasing device is a spring.
 11. The constant force tool of claim 7 wherein, the radially directed force is substantially constant.
 12. The constant force tool of claim 7 wherein, the radially directed force is constant within about 10% of the maximum radially directed force.
 13. A constant force tool comprising, a pivoting force arm, an axial biasing device, wherein the axial biasing device exerts an axially directed force upon the pivoting force arm, further wherein the axially directed force is converted to a radially a directed force, and an extension limiter.
 14. The constant force tool of claim 13 wherein, the extension limiter prevents the pivoting force arms from extending past a predetermined maximum.
 15. The constant force tool of claim 13 further comprising a telescoping arm wherein a first end of the telescoping arm is pivotally attached to the pivoting force arm.
 16. The constant force tool of claim 15 further comprising at least one wheel mounted at the pivotal attachment between the pivoting force arm and the telescoping arm.
 17. The constant force tool of claim 13 wherein, the axial biasing device is a spring.
 18. The constant force tool of claim 13 wherein, the radially directed force is substantially constant.
 19. The constant force tool of claim 13 wherein, the radially directed force is constant within about 10% of the maximum radially directed force.
 20. A constant force tool comprising, a pivoting force arm, an axial biasing device, wherein the axial biasing device exerts an axially directed force upon the pivoting force arm, further wherein the axially directed force is converted to a radially a directed force, and a pivoting force arm lock.
 21. The pivoting force tool of claim 20 wherein, the pivoting force arm lock prevents the pivoting force arms from extending prior to a predetermined event.
 22. The pivoting force arm tool of claim 21 wherein, the predetermined event is exceeding a predetermined pressure differential.
 23. The constant force tool of claim 21 further comprising a telescoping arm wherein a first end of the telescoping arm is pivotally attached to the pivoting force arm.
 24. The constant force tool of claim 23 further comprising a wheel mounted at the pivotal attachment between the pivoting force arm and the telescoping arm.
 25. The constant force tool of claim 21 wherein, the axial biasing device is a spring.
 26. The constant force tool of claim 21 wherein, the radially directed force is substantially constant.
 27. The constant force tool of claim 1 wherein, the radially directed force is constant within about 10% of the maximum radially directed force.
 28. A constant force tool comprising, a first pivoting force arm, a second pivoting force arm, wherein a first end of the first pivoting force arm and a first end of the second pivoting force arm extend toward each other from a first end of a mandrel and from an opposing second end of the mandrel, further wherein the first pivoting force arm is circumferentially offset from the second pivoting force arm, a first axial biasing device that exerts a first axially directed force upon the first pivoting force arm, a second axial biasing device that exerts a second axially directed force upon the second pivoting force arm, a first force guide mounted on the mandrel, wherein the first force guide interacts with the first pivoting force arm to convert the first axially directed force to a substantially constant first radially directed force, and a second force guide mounted on the mandrel, wherein the second force guide interacts with the second pivoting force arm to convert the second axially directed force to a substantially constant second radially directed force.
 29. The constant force tool of claim 28 further comprising a telescoping arm wherein a first end of the telescoping arm is pivotally attached to the first pivoting force arm.
 30. The constant force tool of claim 29 further comprising a wheel mounted at the pivotal attachment between the first pivoting force arm and the telescoping arm.
 31. The constant force tool of claim 28 wherein, the axial biasing device is a spring.
 32. The constant force tool of claim 28 wherein, the substantially constant first radially directed force is constant within about 10% of the maximum first radially directed force.
 33. The constant force tool of claim 28 wherein, the substantially constant second radially directed force is constant within about 10% of the maximum second radially directed force. 